Service cranes are known for use on gated dam spillways. Most large navigation dams incorporate an overhead bridge mounted crane which is used to place dewatering bulkheads and lift gate system componants. In order to minimize the need for complete dewatering of individual gates, tainter gates which are hinged above the downstream waterline are frequently used. Bulkhead slots are provided either upstream only or both upstream and downstream to permit dewatering. Such installations are extremely expensive in large part because of the need for piers and an overhead craneway.
Simple mechanical wickets which are operated from a work boat have a lower initial cost than the tainter gate installations, but are labor intensive to operate. Service for such mechanical wickets, if required, is provided by a boat positioned caisson.
Bottom hinged hydraulic gates can be installed without piers. In this case, it is required that a large area be dewatered without the benefit of piers should the gates require maintentance.
Pneumatic spillway gates such as those described in U.S. Pat. No. 4,780,024 and U.S. Pat. No. 5,092,707 can be economically made in shorter lengths than hydraulic gates, resulting in a smaller size of dewatered area being required for servicing the gates.